1. Field of the Invention
The invention relates to a CDMA transmission system comprising a transmitter in which a data sequence is spread by a code sequence, and a receiver in which the original data sequence is recovered by a detector.
The invention likewise relates to a receiver and a transmitter for a CDMA transmission system.
2. Description of the Related Art
In a transmission system based on Code-Division Multiple Access (CDMA), the signals of different users are simultaneously switched to a common frequency band and transmitted with a common carrier frequency. Code-division multiple access systems are based on a spread band technique, i.e., the signal to be transmitted is spread over a frequency band which is considerably broader than the least required frequency band for a signal transmission. The band spreading renders code-division multiple access systems generally highly resistant to interference.
For frequency band spreading in code-division multiple access systems each (payload) bit to be transmitted is multiplied by a codeword agreed upon by transmitter and receiver. For distinguishing from the bits of the payload data sequence to be coded, a bit of the codeword is generally referenced a chip. The use of suitable codewords precludes, in principle, mutual interference of the signals of the individual users.
For obtaining a specific bit error rate with CDMA technology, at a given interference power the signal power must be approximately proportional to the transmit payload bit rate. In CDMA systems, in which the individual users use different payload bit rates depending on the application, for example, coded speech transmission, facsimile transmission, and so on, it is desirable for reasons of economic use of frequencies to load the CDMA transmission system only with the necessary frequency bandwidth.
For example from WO 92/15164 such a CDMA transmission system is known in which a programmable clock generator is provided for obtaining a variable bandwidth, by means of which generator the chip clock rate of each selected codeword is predefined. The higher the chip clock rate is selected, the broader is the resultant bandwidth of the spread transmit signal.
For practical reasons the chip clock rates are selected to be in multiples of a basic clock rate. In the preferred embodiment of said Patent application the bandwidth is therefore 1.2 MHz or a multiple thereof.